Electrodes are widely used in many devices, for example, primary (non-rechargeable) battery cells, secondary battery cells, fuel cells, and capacitors. Electrodes are typically constructed using two or even more constituent materials. These electrodes are known as composite electrode. One application where composite electrodes are often used is construction of double layer capacitors, which are also known as electrochemical capacitors, supercapacitors, and ultracapacitors.
Double layer capacitors employ electrodes immersed in an electrolytic solution as their energy storage element. Typically, a porous separator soaked in the electrolyte ensures that the electrodes do not come in contact with each other. A double layer of charges is formed at the interface between the solid electrodes and the electrolyte. (Double layer capacitors owe their descriptive name to these layers.) When electric potential is applied between a pair of electrodes, ions that exist within the electrolyte are attracted to the surfaces of the electrodes, and migrate towards the electrodes. A layer of oppositely-charged ions is thus created and maintained near each electrode surface. The electrical energy is stored in the charge separation layers between the ionic layers and the charge layers at the corresponding electrode surfaces. The charge separation layers behave essentially as capacitors.
Additional energy can also be stored in the double layer capacitors because of orientation and alignment of molecules of the electrolytic solution under influence of the electric potential.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for this volumetric and weight efficiency. First, the width of the charge separation layers is very small, on the order of nanometers. Second, the electrodes can be made from a porous material, having very large area per unit volume. Because capacitance is directly proportional to the electrode area, and inversely proportional to the width of the charge separation layer, the combined effect of the narrow charge separation layer and large surface area results in a capacitance that is very high in comparison to that of conventional capacitors. High capacitance of the double layer capacitors allows the capacitors to receive, store, and release large supplies of electrical energy.
Another important performance parameter of a capacitor is its internal resistance. Frequency response of a capacitor depends on the characteristic time constant of the capacitor, which is essentially a product of the capacitance and the internal resistance, or RC. To put it differently, internal resistance limits both charge and discharge rates of a capacitor, because the resistance limits the current that flows into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications. In automotive applications, for example, a capacitor that is used as the energy storage element that powers a vehicle's engine has to be able to provide high instantaneous power during acceleration, and to receive bursts of power produced by regenerative braking. In internal combustion-powered vehicles, the capacitor periodically powers a vehicle's starter, also requiring high power in relation to the size of the capacitor.
The internal resistance also creates heat during both charge and discharge cycles. Heat causes mechanical stresses and speeds up various chemical reactions, thereby accelerating capacitor aging. It is therefore desirable to reduce internal resistance of capacitors. Moreover, the energy converted into heat is lost, decreasing the efficiency of the capacitor.
Active materials used for electrode construction—activated carbon, for example—usually have rather limited specific conductance. Thus, large contact area may be desired to minimize the contact resistance between the electrode and its terminal. The active material may also be too brittle or otherwise unsuitable for directly connecting to terminals. Additionally, the material may have relatively low tensile strength, needing mechanical support in some applications. For these reasons, electrodes incorporate current collectors.
A current collector is typically a sheet of conductive material on which the active electrode material is deposited, either directly or over one or more intermediate layers. Often, aluminum foil is used as the current collector material of a composite electrode. In one electrode fabrication process, a film that includes activated carbon powder (i.e., the active electrode material) is produced, and then attached to a thin aluminum foil using an adhesive. The use of the adhesive improves bonding of the active electrode material to the current collector. Unfortunately, this process also has a number of disadvantages.
First, the adhesive increases the cost of materials consumed in the process of electrode fabrication; some adhesives are quite expensive.
Second, two steps are added to the fabrication process. The adhesive must be applied onto the current collector foil, or onto the active electrode film. The adhesive must also be allowed to dry and cure. These extra steps increase the cost of the final product.
Third, the adhesive may deteriorate with time, contributing to an increase in the internal resistance of the electrode. In some double layer capacitors, for example, the electrolyte reacts chemically with the adhesive, causing the adhesive to weaken and the bond created by the adhesive to fail.
Fourth, adhesive use reduces the energy storage efficiency of the electrode, because the adhesive penetrates into the pores of the active electrode material, decreasing the total surface active area of the electrode. Therefore, it would be preferable to reduce or eliminate the use of adhesives in compound electrodes.
A need thus exists for methods for fabricating compound electrodes without the use of adhesives at the interface between the active electrode material and the current collector. A further need exists for electrodes fabricated without the use of adhesives at this interface. Still another need exists for energy storage devices with electrodes without adhesive on the interfaces between the active layers and the current collectors.